Based on biochemical evidence, a high prevalence of biochemical evidence of vitamin B12 or folate deficiency has been reported in a number of areas in the world. The evidence that these biochemical abnormalities lead to a comparable prevalence of anemia is reviewed. The overall contribution of vitamin B12 deficiency to the global burden of anemia is probably not significant, except perhaps in women and their infants and children in vegetarian communities. In developed countries, folate-deficiency anemia is uncommon. In some developing countries, this anemia is still seen, but there are no comprehensive data on the relative prevalence compared with anemia due to malaria, iron-deficiency, hemoglobinopathy, and HIV disease. It seems unlikely that folate deficiency makes a major contribution to the burden of anemia in developing countries. Iron-deficiency anemia may coexist with vitamin B12 and especially folate deficiency, and may confound the hematological features of the vitamin deficiencies whose prevalence would then be underestimated. Supplementation of the diet of pregnant women with folic acid can virtually eliminate folate-deficiency anemia in these women. There are very few data on the hematological effect of vitamin B12 supplementation or fortification at the population level. The addition of vitamin B12 to the supplementation of the diet of pregnant women with iron and folic acid does not produce an increased hematological response, at least in nonvegetarian populations. There are numerous reports of the effect of folic acid fortification of food on tests of folate status, but only a single published report on the hematological response was found.